User blog comment:TheSinnohDuo/What should i EV train my quiet natured Prinplup in? (will be an Empoleon)/@comment-32358261-20180217114802

Empoleon is used as a bulky water type ultility pokemon. So invest in sp def, hp and maybe a little in sp atk (for a bit more power).

Or go completely defensive and go for def ev investment. Also best nature for it would be calm.